Photohardenable electrostatic element with improved environmental latitude

ABSTRACT

Xeroprinting process comprising 
     (A) exposing imagewise to actinic radiation a photohardenable electrostatic master comprising 
     (1) an electrically conductive substrate, and 
     (2) a layer of photohardenable composition consisting essentially of 
     (a) at least two organic polymeric binders, at least one having a Tg greater than 80° C. and at least one having a Tg less than 70° C., 
     (b) at least one monomeric compound having at least one ethylenically unsaturated group, and 
     (c) a photoinitiator or photoinitiator system, the shift in transit time (a T ) of the photohardenable layer in the range of 30%≦relative humidity≦60% and 15.6° C.≦temperature ≦26.7° C. is 15 or less, preferably 10 or less 
     (B) charging the photohardenable master electrostatically, 
     (C) applying an oppositely charged electrostatic toner, and 
     (D) transferring the toned image to a receptor surface. The xenoprinting process uses the master in graphic arts, color proofing which duplicates images produced by printing, preparation of printed circuit boards, resists, soldermasks, etc.

This is a division of application Ser. No. 07/351,361, filed May 12,1989, now U.S. Pat. No. 5,006,434.

DESCRIPTION FIELD OF THE INVENTION

This invention relates to xeroprinting process. More particularly thisinvention relates to a xeroprinting process using a photohardenableelectrostatic master having on an electrically conductive substrate alayer of a photohardenable composition which contains at least twopolymeric binders, at least one binder having a relatively high glasstransition temperature and at least one binder having a relatively lowglass transition temperature.

BACKGROUND OF THE INVENTION

The xeroprinting process employs a printing plate, commonly referred toas a "master", made by creating a pattern of insulating material (i.e.,an image) on the surface of a grounded conductive substrate. In thexeroprinting process, an electrostatic charge is applied to the surfaceof the master, e.g., by corona discharge. The portion of the masterbearing the insulating material retains the charge, while the charge onthe remainder of the master is discharged through the groundedconductive substrate. Thus, a latent image of electrostatic charge isformed on the insulating material, the image subsequently beingdeveloped with either oppositely charged particles commonly referred toas "toner" or liquid electrostatic developers. The toner is thentransferred (e.g., by electrostatic or other means) to another surface(e.g., paper or polymeric film), where it is fused (i.e., "fixed"), toreproduce the image of the master. Since the image on the master ispermanent, or at least persistent, multiple copies can be made byrepeating the charging, toning and transfer steps.

Recently issued U.S. Pat. No. 4,732,831 to Riesenfeld et al. disclosesan improved xeroprinting process that employs a master having aphotopolymerizable coating on a conducting substrate. The coatingcontains an organic polymeric binder, an ethylenically unsaturatedmonomer, and a photoinitiator system. When the master is exposed to thedesired pattern of actinic radiation (i.e., light of a suitablewavelength), exposed regions of the coating polymerize and exhibit asignificantly higher electrical resistance than unexposed regions. Thus,when the master is subsequently used in the xeroprinting process, thepolymerized regions will hold an electrical charge, which is developedwith toner, while the unpolymerized regions discharge to ground throughthe conductive backing and therefore do not attract the toner.

It has been found that the electrostatic properties ofphotopolymerizable masters change considerably with small variations inambient temperature around room temperature (RT). Relatively smallchanges in humidity at these temperature conditions also affectselectrostatic properties. For example, the discharge rates of thephotopolymerizable layer increase with a rise in temperature. Changes inthe discharge rate with ambient temperature result in degradation ofprint quality as well as unacceptable dot gain and dot range. Lowertemperatures (RT-5° C.) show lack of shadow dots while at highertemperatures (RT+5° C.) highlight dots and dot gains diminish.

There has been no attempt to understand the change in resistivity uponexposure of a photopolymerizable element or the details of theconduction mechanism. Transport of charge in photopolymerizableelements, however, has been studied. The photopolymerizableelectrostatic master when unexposed has its glass transition temperature(Tg) near ambient temperature. The electrical conductivity is the resultof conduction by impurity ions with a reasonable degree of mobility inthe liquid-like photopolymerizable layer of high viscosity. Uponimagewise exposure, the Tg of the photopolymerized layer shifts closerto the Tg of the binder component, e.g., about 100° C., and the iphotopolymerized areas are in a glassy state. The electricalconductivity decreases several orders of magnitude corresponding to thechange in charge mobility in the photopolymerized (glassy) areas. Themodification of the electrical properties of the photopolymerizableelectrostatic master by change in glass transitions made one havingordinary skill in the art believe that the change in Tg affected thetemperature and humidity sensitivity of the master as well. Based onthis assumption it appeared unlikely that photopolymerizableelectrostatic masters would be achieved that were substantiallyinsensitive to the changes in the environment, e.g., temperature andhumidity.

It has now been found that a photopolymerizable electrostatic masterhaving improved environmental latitude can be made wherein the abovedisadvantages are substantially overcome by introducing into thephotopolymerizable composition forming the photopolymerizable layer ablend of binders, at least one binder having a relatively higher glasstransition temperature than at least one other binder present. Theimproved photopolymerizable electrostatic master exhibits good imagequality, electrical properties and temperature stability.

SUMMARY OF THE INVENTION

In accordance with this invention there is provided an improvedphotohardenable electrostatic master having reduced temperature andhumidity sensitivity comprising

(1) an electrically conductive substrate, and

(2) a layer of photohardenable composition consisting essentially of

(a) at least two organic polymeric binders,

(b) at least one monomeric compound having at least one ethylenicallyunsaturated group, and

(c) a photoinitiator or photoinitiator system that activatepolymerization of the ethylenically unsaturated monomer upon exposure toactinic radiation, the improvement wherein the photohardenablecomposition contains at least one binder having a Tg greater than 80° C.and at least one binder having a Tg less than 70° C. such that the shiftin transit time (a_(T)) of the photohardenable layer in the range 30%≦relative humidity≦60% and 60° F. (15.6° C.)≦temperature≦80° F. (26.7°C.) is 15 or less.

In accordance with an embodiment of this invention there is provided axeroprinting process comprising

(A) exposing imagewise to actinic radiation a photohardenableelectrostatic master comprising

(1) an electrically conductive substrate, and

(2) a layer of photohardenable composition consisting essentially of

(a) at least two organic polymeric binders,

(b) at least one monomeric compound having at least one ethylenicallyunsaturated group, and

(c) a photoinitiator or photoinitiator system that activatespolymerization of the ethylenically unsaturated monomer upon exposure toactinic radiation, the photohardenable composition containing at leastone binder having a Tg greater than 80° C. and at least one binderhaving a Tg less than 70° C. such that the shift in transit time (a_(T))of the photohardenable layer in the range 30%≦relative humidity≦60% and60° F. (15.6° C.) ≦temperature≦80° F. (26.7° C.) is 15 or less.

(B) charging the photohardenable master electrostatically,

(C) applying an oppositely charged electrostatic toner, and

(D) transferring the toned image to a receptor surface.

DETAILED DESCRIPTION OF THE INVENTION

Throughout the specification the below-listed terms have the followingmeanings:

In the claims appended hereto "consisting essentially of" means thecomposition of the photohardenable layer does not exclude unspecifiedcomponents which do not prevent the advantages of the layer from beingrealized. For example, in addition to the primary components, there canbe present additional components, such as sensitizers, including visiblesensitizers, hydrogen donors or chain transfer agents (preferred), bothof which are considered part of the photoinitiator system; thermalstabilizers or thermal polymerization inhibitors, photoinhibitors,antihalation agents, UV absorbers, release agents, colorants,surfactants, plasticizers, electron donors, electron acceptors, chargecarriers, etc.

Photohardenable and photopolymerizable are used interchangeably in thisinvention.

Glass transition temperature (Tg) is the main characteristic temperatureabove which the amorphous polymer acquires sufficient thermal energy andchanges from a glassy to a rubbery state accompanied by significantchanges in physical properties due to facilitated molecular motion.

Monomer means simple monomers, as well as polymers, usually of molecularweights below 1500, having at least one, preferably two or more,ethylenic groups capable of crosslinking or addition polymerization.

Photopolymerizable layers of this invention having improvedenvironmental latitude have broadened glass transition temperature inthe unexposed state with respect to such layers having a single binder.The glass transition range is broadened by introducing into theformulation a blend of binders having high and low Tg's. Blends ofmonomers have been found to further improve environmental latitude. Thebinder mixture consists of at least two materials with different glasstransition temperatures. In general, it has been found that a high Tgbinder (approximately in the range of 80-110° C. and a low Tg binder(approximately in the range of 50-70° C.) are preferred. The molecularweights of the low Tg binders were found not to have a noticeable effectin the temperature stability of the photohardenable composition andmainly modified coating properties.

The primary components include:

BINDERS

Suitable binders include: acrylate and methacrylate polymers and co- orterpolymers, vinyl polymers and copolymers, polyvinyl acetals,polyesters, polycarbonates, polyurethanes, polysulfones, polyetherimidesand polyphenylene oxides, butadiene copolymers, cellulose esters,cellulose ethers, etc. The selection of a polymeric binder depends onits Tg. The Tg of a polymer is affected by the chemical structures ofthe main chain and the side groups. Polymers with rigid structuresgenerally show high Tg's while more flexible polymers exhibit low Tg's.Polymers of desired Tg's may be obtained by copolymerization of propercombinations of rigid and flexible monomers. The following publicationwhich summarizes glass transition temperatures of homopolymers known inthe literature, "POLYMER HANDBOOK", ed. J. Brandrup & E. H. Immergut,John Wiley & Sons, Inc., 1975, is incorporated herein by reference.Section III-140-192 of said publication lists Tg's of most knownpolymers.

Examples of useful binders having Tg's greater than 80° C. include:

    ______________________________________                                        TRADE NAME                                                                    OR CODE     CHEMICAL COMPOSITION                                                                              Tg (°C.)                               ______________________________________                                        Vinyl polymers & copolymers                                                   PSMMA       Poly(styrene(70)/methyl                                                                            95                                                       methacrylate(30))                                                 Cycolac ® CTB                                                                         Acrylonitrile/butadiene/styrene                                                                   80-84                                         (Borg-Warner)                                                                             Polystyrene         100                                                       Poly(alpha-methylstyrene)                                                                         168                                                       Poly(vinyl chloride)                                                                               80                                                       Poly(vinylidene chloride)                                                                         100                                                       Poly(acrylonitrile)  96                                           Methacrylate polymers & copolymers                                                      Poly(methyl methacrylate)                                                                       110                                                         Poly(isobornyl methacrylate)                                                                    147                                                         Poly(phenyl methacrylate)                                                                       110                                                         Poly(t-butyl methacrylate)                                                                      107                                                         Poly(isopropyl methacrylate)                                                                     81                                               Condensation polymers                                                         Lexan ® 101 (G.E.)                                                                    Polycarbonate       150                                                       Polysulfone         190                                           ULTEM ® (G.E.)                                                                        Polyetherimide      215                                                       Poly(phenylene oxide)                                                                             210                                                       Poly(1,4-Cyclohexanedimethanol                                                                     85                                                       terephthalate)                                                    Polyvinyl acetals                                                                         Poly(vinyl acetal)   83                                           Formvar ® (Mon-                                                                       Poly(vinyl formal)   92-113                                       santo)                                                                        ______________________________________                                    

Examples of useful binders having Tg's less than 70° C. include:

    ______________________________________                                        TRADE NAME                                                                    OR CODE     CHEMICAL COMPOSITION                                                                              Tg (°C.)                               ______________________________________                                        Acrylate, methacrylate polymers & copolymers                                              Poly(ethyl methacrylate)                                                                          70                                            Elvacite ® 2042                                                                       Poly(ethyl methacrylate)                                                                          65                                            Elvacite ® 2045                                                                       Poly(isobutyl methacrylate)                                                                       55                                            Elvacite ® 2014                                                                       Methyl methacrylate copolymer                                                                     40                                            Elvacite ® 2044                                                                       Poly(n-butyl methacrylate)                                                                        15                                            Elvacite ® 2046                                                                       Poly(n-butyl/isobutyl                                                                             35                                                        methacrylate)                                                     (E. I. du Pont                                                                            Poly(cyclohexyl methacrylate)                                                                     66                                            de Nemours & Co.)                                                                         Poly(t-butyl acrylate)                                                                            41                                            Vinyl esters & copolymers                                                               Poly(vinyl acetate)                                                                             32                                                Vinyl polymers & copolymers                                                               Vinyl chloride/vinyl acetate                                                                      63                                                        copolymer                                                         Polyvinyl acetals                                                             Butvar ® (Mon-                                                                        Poly(vinyl butyral) 62-68                                         santo)                                                                        Polyurethanes                                                                 Estane ® 5715                                                                         Polyurethane        16                                            (B.F. Goodrich)                                                               Polyesters                                                                                Poly(tetramethylene 45                                                        terephthalate)                                                    Butadiene copolymers                                                                    Styrene/butadiene <70                                                         copolymers                                                          Cellulose esters and ethers                                                             Ethyl cellulose   43                                                ______________________________________                                    

Preferred binders include the Elvacite® resins because their Tg's rangefrom 15° C. to 105° C. Low Tg resins include poly(ethyl methacrylate)(Tg 70° C.), Elvacite®2045 or 2042, in combination with high Tg resinspoly(methyl methacrylate) (Tg 110° C.) or poly(styrene/methylmethacrylate) are particularly preferred. The binder combination ofpoly(ethyl methacrylate) (Tg 70° C.) and poly(styrene/methylmethacrylate) gave photopolymerizable compositions with goodenvironmental response and coating properties.

The mixed binders should have a resistivity in the range of 10¹⁴ to 10²⁰ohm-cm, preferably 10¹⁴ to 10¹⁶ ohm-cm.

MONOMERS

Any ethylenically unsaturated photopolymerizable or photocrosslinkablecompounds suitable for use with hexaarylbiimidazole initiator systemscan be used in the practice of this invention.

Preferred monomers which have at least two terminally ethylenicallyunsaturated groups are di-, tri-, and tetraacrylates and methacrylatessuch as ethylene glycol diacrylate, diethylene glycol diacrylate,triethylene glycol diacrylate, glycerol diacrylate, glyceroltriacrylate, glycerol propoxylated triacrylate, ethylene glycoldimethacrylate, 1,2-propanediol dimethacrylate, 1,2,4-butanetrioltrimethacrylate, 1,4-cyclohexanediol diacrylate, 1,4-benzenedioldimethacrylate, pentaerythritol triacrylate, pentaerythritoltrimethacrylate, pentaerythritol tetramethacrylate, 1,3-propanedioldiacrylate, 1,5-pentanediol dimethacrylate, trimethylolpropanetriacrylate, ethoxylated trimethylolpropane triacrylate, thebisacrylates and bismethacrylate of polyethylene glycols of molecularweight 100-500, tris-(2-hydroxyethyl)isocyanurate triacrylate, etc.Especially preferred monomers are glyceryl propoxylated triacrylate,trimethylolpropane triacrylate and tris-(2-hydroxyethyl)isocyanauratetriacrylate.

A monomer with a resistivity in the range of about 10⁵ to 10⁹ ohm-cm isparticularly useful. Mixtures of monomers have also been found toenhance the improvement in environmental stability of thephotohardenable or photopolymerizable master. Blends of glycerolpropoxylated triacrylate and trimethylolpropane triacrylate in a 2:1ratio were found to give the best overall performance. Other monomerblends, such as tris-(2-hydroxyethyl) isocyanurate triacrylate andtrimethylolpropane triacrylate show good temperature stability.

INITIATORS AND/OR INITIATOR SYSTEMS

A large number of free-radical generating compounds can be utilized inthe photopolymerizable compositions. Preferred initiator systems are2,4,5-triphenylimidazolyl dimers with hydrogen donors, also known as the2,2',4,4',5,5'-hexaarylbiimidazoles, or HABI's, and mixtures thereof,which dissociate on exposure to actinic radiation to form thecorresponding triarylimidazolyl free radicals. HABI's and use ofHABI-initiated photopolymerizable systems for applications other thanfor electrostatic uses have been previously disclosed in a number ofpatents. These include Chambers, U.S. Pat. No. 3,479,185, Chang et al.,U.S. Pat. No. 3,549,367, Baum and Henry, U.S. Pat. No. 3,652,275,Cescon, U.S. Pat. No. 3,784,557, Dueber, U.S. Pat. No. 4,162,162,Dessauer, U.S. Pat. No. 4,252,887, Chambers et al., U.S. Pat. No.4,264,708, Wada et al. U.S. Pat. No. 4,410,621, and Tanaka et al., U.S.Pat. No. 4,459,349, the disclosures of which are incorporated herein byreference. Useful 2,4,5-triarylimidazolyl dimers are disclosed in Baumand Henry, U.S. Pat. No. 3,652,275 column 5, line 44 to column 7, line16, the disclosure of which is incorporated herein by reference. Any2-o-substituted HABI disclosed in the prior patents can be used in thisinvention.

The HABI's can be represented by the general formula ##STR1## where theR's represent aryl, e.g., phenyl, naphthyl, radicals. The2-o-substituted HABI's are those in which the aryl radicals at the 2-and 2'-positions are ortho-substituted or with polycyclic condensed arylradicals. The other positions on the aryl radicals can be unsubstitutedor carry any substituent which does not interfere with the dissociationof the HABI upon exposure or adversely affect the electrical or othercharacteristics of the photopolymer system.

Preferred HABI's are 2-o-chlorosubstituted hexaphenylbiimidazoles inwhich the other positions on the phenyl radicals are unsubstituted orsubstituted with chloro, methyl or methoxy. The most preferredinitiators include 2-o-chlorophenyl)-4,5-bis(m-methoxyphenyl)imidazoledimer, 1,1'-biimidazole,2,2'-bis(o-chlorophenyl)-4,4,'5,5'-tetraphenyl-, and 1H-imidazole,2,5-bis(o-chlorophenyl)-4-[3,4-dimethoxyphenyl]-, dimer, each of whichis typically used with a hydrogen donor or chain transfer agentdescribed below.

Photoinitiators that are also useful in the photohardenable compositionin place of the HABI type photoinitiators include: the substituted orunsubstituted polynuclear quinones, aromatic ketones, and benzoinethers. Examples of such other photoinitiators are quinones, forexample, 9,10-anthraquinone, 1-chloroanthraquinone,2-chloroanthraquinone, 2-methylanthraquinone, 2-ethylanthraquinone,2-tert-butylanthraquinone, octamethylanthraquinone, 1,4-naphthoquinone,9,10-phenanthrenequinone, 1,2-benzanthraquinone, 2,3-benzanthraquinone,2-methyl-1,4-naphthoquinone, 2,3-dichloronaphthoquinone,1,4-dimethylanthraquinone, 2,3-dimethylanthraquinone,2-phenylanthraquinone, 2,3-diphenylanthraquinone, sodium salt ofanthraquinone alpha-sulfonic acid, 3-chloro-2-methylanthraquinone,retenequinone, 7,8,9,10-tetrahydronaphthacenequinone,1,2,3,4-tetrahydrobenz(a)anthracene-7,12-dione; aromatic ketones, forexample, benzophenone, Michler's ketone,4,4'-bis(dimethylamino)benzophenone; 4,4'-bis(diethylamino)benzophenone,4-acryloxy-4'-diethylaminobenzophenone,4-methoxy-4'-dimethylaminobenzophenone, xanthones, thioxanthones; andbenzoin ethers, for example, benzoin methyl and ethyl ethers. Stillother photoinitiators which are also useful, are described in U.S. Pat.No. 2,760,863 and include vicinal ketaldonyl alcohols, such as benzoin,pivaloin, acyloin ethers, alpha-hydrocarbonsubstituted aromaticacyloins, including alphamethylbenzoin, alpha-allylbenzoin andalphaphenylbenzoin. Additional systems include alphadiketones withamines as disclosed in Chang, U.S. Pat. No. 3,756,827, and benzophenonewith p-dimethylaminobenzaldehyde or with esters ofp-dimethylaminobenzoic acid as disclosed in Barzynski et al., U.S. Pat.No. 4,113,593.

Redox systems, especially those involving dyes, e.g., Rose Bengal®2-dibutylaminoethanol, are also useful in the practice of thisinvention. Photoreducible dyes and reducing agents such as thosedisclosed in U.S. Pat. Nos. 2,850,445; 2,875,047; 3,097,096; 3,074,974;3,097,097; 3,145,104; and 3,579,339; as well as dyes of the phenanzine,oxazine, and quinone classes can be used to initiatephotopolymerization, the disclosures of which are incorporated herein byreference. A useful discussion of dye sensitized photopolymerization canbe found in "Dye Sensitized Photopolymerization" by D. F. Eaton in Adv.in Photochemistry, Vol. 13, D. H. Volman, G. S. Hammond, and K.Gollinick, eds., Wiley-Interscience, New York, 1986, pp. 427-487.

SENSITIZERS

Sensitizers useful with these photoinitiators include those disclosed inU.S. Pat. Nos. 3,554,753; 3,563,750; 3,563,751; 3,647,467; 3,652,275;4,162,162; 4,268,667; 4,351,893; 4,454,218; 4,535,052; and 4,565,769,the disclosures of which are incorporated hereby by reference.

A preferred group of visible sensitizers include thebis(p-dialkylaminobenzylidene) ketones disclosed in Baum and Henry, U.S.Pat. No. 3,652,275 and the arylyidene aryl ketones disclosed in Dueber,U.S. Pat. No. 4,162,162, as well as in U.S. Pat. Nos. 4,268,667 and4,351,893, the disclosure of each being incorporated herein byreference. These compounds extend the sensitivity of the initiatorsystem to visible wavelengths where lasers emit. Particularly preferredsensitizers are DMJDI:2-{9'-(2',3',6',7'-tetrahydro-1H,5H-benzo[i,j]-quinolylideene}-5,6-dimethoxy-1-indanone(DMJDI) and JAW:2,5-Bis{9'-(2',3',6',7'-tetrahydro-1H,5H-benzo[i,j]-quinolylidene)}cyclopentanone(JAW) which have the following structures: ##STR2##

CHAIN TRANSFER AGENTS

Any chain transfer agent, or hydrogen donor, identified in the priorpatents for use with HABI-initiated photopolymerizable systems can beused. For example, Baum and Henry, U.S. Pat. No. 3,652,275 disclosesN-phenylglycine, 1,1-dimethyl-3,5-diketocyclohexane, and organic thiolssuch as 2-mercaptobenzothiazole, 2-mercaptobenzoxazole,2-mercaptobenzimidazole, pentaerythritol tetrakis(mercaptoacetate),4-acetamidothiophenol, mercaptosuccinic acid, dodecanethiol, andbetamercaptoethanol, 2-mercaptoethane sulfonic acid,1-phenyl-4H-tetrazole-5-thiol, 6-mercaptopurine monohydrate,bis-(5-mercapto-1,3,4-thiodiazol-2-yl, 2-mercapto-5-nitrobenzimidazole,and 2-mercapto-4-sulfo-6-chlorobenzoxazole, the disclosure of which isincorporated by reference. Also useful are various tertiary amines knownin the art. Other hydrogen donor compounds useful as chain transferagents in photopolymer compositions include various other types ofcompounds, e.g., (a) ethers, (b) esters, (c) alcohols, (d) compoundscontaining allylic or benzylic hydrogen cumene, (e) acetals, and (f)aldehydes, as disclosed in column 12, lines 18 to 48, of MacLachlan,U.S. Pat. No. 3,390,996, the disclosure of which is incorporated hereinby reference. The preferred chain transfer agents are2-mercaptobenzoxazole (2-MBO) and 2-mercaptobenzimidazole (2-MBI).

ADDITIONAL COMPONENTS

The photohardenable compositions may also contain other ingredientswhich are conventional components used in photopolymerizable systems.Such components include thermal stabilizers or thermal polymerizationinhibitors, photoinhibitors, antihalation agents, UV absorbers, releaseagents, colorants, surfactants, plasticizers, electron donors, electronacceptors, charge carriers, etc.

Normally a thermal polymerization inhibitor will be present in smallquantities, e.g.,<0.1%, to increase stability in the storage of thephotopolymerizable composition. Useful thermal polymerization inhibitorsor thermal stabilizers include: hydroquinone, phenidone,p-methoxyphenol, alkyl and aryl-substituted hydroquinones and quinones,tert-butyl catechol, pyrogallol, copper resinate, naphthylamines,beta-naphthol, cuprous chloride, 2,6-di-tert-butyl p-cresol,phenothiazine, pyridine, nitrobenzene, dinitrobenzene, p-toluquinone andchloranil. The dinitroso dimers described in Pazos, U.S. Pat. No.4,168,982 are also useful, the disclosure of which is incorporated. Thepreferred stabilizer is TAOBN, i.e.,1,4,4-trimethyl-2,3-diazobicyclo-(3.2.2)-non-2-ene-N,N-dioxide.Photoinhibitors are disclosed in Pazos U.S. Pat. No. 4,198,242, thedisclosure of which is incorporated herein by reference. A specificphotoinhibitor is1-(2'-nitro-4',5'-dimethoxy)phenyl-1-(4-t-butylphenoxy)ethane.

Antihalation agents useful in the photohardenable compositions includeknown antihalation dyes.

Ultraviolet radiation absorbing materials useful in the invention arealso disclosed in U.S. Pat. No. 3,854,950, the disclosure of which isincorporated herein by reference.

Compounds present in the composition as release agents are described inBauer, U.S. Pat. No. 4,326,010, the disclosure of which is incorporatedherein by reference. A specific release agent is polycaprolactone.

Suitable plasticizers include: triethylene glycol, triethylene glycoldipropionate, triethylene glycol dicaprylate, triethylene glycolbis(2-ethyl hexanoate), tetraethylene glycol diheptanoate, polyethyleneglycol, diethyl adipate, tributyl phosphate, etc. Other plasticizersthat yield equivalent results will be apparent to those skilled in theart.

Suitable electron donors and acceptors are disclosed in Blanchet-Fincheret al., U.S. Ser. No. 07/116,655, filed Nov. 4, 1987, the disclosure ofwhich is incorporated herein by reference.

Suitable charge carriers are disclosed in Blanchet-Fincher et al. U.S.Pat. No. 4,818,660, the disclosure of which is incorporated herein byreference.

PROPORTIONS

In general, the components should be used in the following approximateproportions: binder 40-70%, preferably 50-65%; monomer 20-40%,preferably 20-35%, initiator 1-20%, preferably 1-8%, and chain transferagent or hydrogen donor 0-10%, preferably 0.1-4%. These are weightpercentages based on total weight of the photopolymerizable system.

The preferred proportions depend upon the particular compounds selectedfor each component and the application for which the photohardenablecomposition is intended. For example, a high conductivity monomer can beused in smaller amount than a low conductivity monomer, since the formerwill be more efficient in eliminating charge from unexposed areas.

The amount of HABI will depend upon film speed requirement.Photohardenable compositions with HABI content above 10% provide filmsof high sensitivity (high speed) and can be used with laser imaging inrecording digitized information, as in digital color proofing. Suchfilms are the subject of Legere U.S. Ser. No. 07/284,861, filed Dec. 13,1988. For analog applications, e.g., exposure through a negative, filmspeed requirement depends upon mode of exposure. Slow speed films areacceptable for analog applications.

COATING/SUBSTRATES

The photohardenable layer is prepared by mixing the ingredients of thephotopolymerizable composition in a solvent, such as methylene chloride,usually in the weight ratio of about 15:85 to 25:75 (solids to solvent),coating on a substrate, and evaporating the solvent. Coatings should beuniform and should have a thickness of 3 to 20 μm, preferably 7 to 12μm, when dry. Dry coating weight should be about 30 to 200 mg/dm²,preferably 80 to 150 mg/dm². A coversheet, e.g., polyethylene,polypropylene, polyethylene terephthalate, etc. is preferably placedover the photohardenable layer after the solvent evaporates forprotection.

The substrate should be uniform and free of defects such as pinholes,bumps, and scratches. It can be a support, such as paper, glass,synthetic resin and the like, which has been coated by vapor depositionor sputtering chemical deposition on one or both sides with a metal,conductive metal oxide, or metal halide, such as aluminized polyethyleneterephthalate; or a conductive paper or polymeric film. The coatedsubstrate mounted directly on a conductive support can be mounteddirectly on the printing device.

Alternatively, the substrate can be a non-conducting film, preferably arelease film such as polyethylene or polypropylene. After removal of theprotective cover sheet, the photohardenable layer can then be laminatedto a conductive support on the printing device with the tacky,photohardenable layer adjacent to the support. The substrate then actsas a coversheet which is removed after exposure but prior to charging.

As another alternative, the conductive support may be a metal plate,such as aluminum, copper, zinc, silver or the like; or a support whichhas been coated with a polymeric binder containing a metal, conductivemetal oxide, metal halide, conductive polymer, carbon, or otherconductive filler.

ELECTRICAL CHARACTERISTICS

To evaluate the photopolymerizable compositions, voltage is measured onthe unexposed photohardenable layer as a function of time using standardconditions of charging and measurement.

The desired electrical properties of the photohardenable element aredependent on the charge deposited on the photohardenable surface and theelectrical characteristics of the particular toner system employed.Ideally, at the time of contact, e.g., with a developer dispersion, thevoltage in the exposed areas (Vexp) should be at least 10 V, preferablyat least 100 V and even up to 400 V or higher, more than that of thevoltage in unexposed areas (Vunexp). Resistivity of the exposed areasshould be between about 10¹⁴ and 10¹⁷ ohm-cm. Resistivity in theunexposed areas should be between 10¹² and 10¹⁵ ohm-cm and the ratio ofresistivity in exposed areas to resistivity in unexposed areas should beat least 100. A typical time for toner or developer application isbetween 1 and 5 seconds after charging.

EXPOSURE/CHARGING/TONING/TRANSFER

To provide the required conductivity differential, exposure must besufficient to cause substantial polymerization in exposed areas.Exposing radiation can be modulated by either digital or analog means.Analog exposure utilizes a line or halftone negative or other patterninterposed between the radiation source and film. For analog exposure anultraviolet light source is preferred, since the photopolymerizablesystem is most sensitive to shorter wavelength radiation. Digitalexposure may be carried out by a computer controlled, light-emittinglaser which scans the film in raster fashion. For digital exposure ahigh speed film, i.e., one which contains a high level of HABI and whichhas been sensitized to longer wavelengths with a sensitizing dye, ispreferred. Electron beam exposure can be used, but is not preferredbecause of the expensive equipment required.

The preferred electrostatic charging means is corona discharge. Othercharging methods include: discharge of a capacitor, negative coronadischarge, shielded corotron, scorotron, etc.

Any electrostatic toner or developer and any method of developerapplication can be used. Liquid toners, i.e., a suspension of pigmentedresin toner particles in a nonpolar dispersant liquid present in majoramount, are preferred. The liquids normally used are Isopar®branched-chain aliphatic hydrocarbons (sold by Exxon Corporation) whichhave a Kauri-butanol value of less than 30. These are narrow high-puritycuts of isoparaffinic hydrocarbon fractions with the following boilingranges Isopar®-G, 157-176° C., Isopar®-H 176-191° C., Isopar®-K 177-197°C., Isopar®-L 188-206° C., Isopar®-M 207-254° C., Isopar®-V 254-329° C.The liquid developers may contain various adjuvants which are describedin: Mitchell, U.S. Pat. Nos. 4,631,244, 4,663,264, and 4,734,352; Taggi,U.S. Pat. No. 4,670,370; El-Sayed and Taggi, U.S. Pat. No. 4,702,984;Larson, U.S. Pat. No. 4,702,985; Trout, U.S. Pat. No. 4,707,429; Larsonand Trout U.S. Pat. No. 4,681,831. The liquid electrostatic developerscan be prepared as described in Larson U.S. Pat. No. 4,760,009. Thedisclosures in these patents are incorporated herein by reference.

Also present in the liquid electrostatic developers are thermoplasticresins, having an average particle size of less than 10 μm, which are,for example, copolymers Of ethylene 80 to 99.9 %) with acrylic acid,methacrylic acid, or alkyl esters, where alkyl is 1 to 5 carbon atoms,of acrylic or methacrylic acid (20 to 0.1%), e.g., anethylene/methacrylic acid (89:11) copolymer having a melt index at 190°C. of 100. Preferred nonpolar liquid soluble ionic or zwitterioniccomponents present in such developers, for example, are lecithin andBasic Barium Petronate® oil-soluble petroleum sulfonate, Whitco Chem.Corp., New York, N.Y.

Many of the monomers useful in the photohardenable composition describedabove are soluble in these Isopar® hydrocarbons, especially inIsopar®-L. Consequently, repeated toning with Isopar®-based developersto make multiple copies can deteriorate the electrical properties of thephotohardenable master by extraction of monomer from unexposed areas.The preferred monomers are relatively insoluble in Isopar® hydrocarbons,and extended contact with these liquids does not unduly deterioratephotohardenable layers made with these monomers. Photohardenableelectrostatic masters made with other, more soluble monomers can stillbe used to make multiple copies, using liquid developer having adispersant with less solvent action.

Representative dry electrostatic toners that may be used include: KodakEktaprint K, Hitachi HI-Toner HMT-414, Canon NP-350F toner, ToshibaT-50P toner, etc.

After developing the toned image is transferred to a receptor surface,such as paper, for the preparation of a proof. Other receptors arepolymeric film, or cloth. For making integrated circuit boards, thetransfer surface can be an insulating board on which conductive circuitlines can be printed by the transfer, or the surface can be aninsulating board covered with a conductor, e.g., a fiber glass boardcovered with a copper layer, on which a resist is printed by transfer.

Transfer is accomplished by electrostatic or other means, e.g., bycontact with an adhesive receptor surface. Electrostatic transfer can beaccomplished in any known manner, e.g., by placing the receptor surface,e.g., paper, in contact with the toned image. A tackdown roll or corona,when held at negative voltages, will press the two surfaces togetherassuring intimate contact. After tackdown, a positive corona dischargeis applied to the backside of the paper to drive the toner particles offthe electrostatic master onto the paper.

INDUSTRIAL APPLICABILITY

The photohardenable electrostatic master having improved environmentallatitude is particularly useful in the graphic arts field, especially inthe area of color proofing wherein the proofs prepared duplicate theimages produced by printing. This is accomplished by controlling thegain of the reproduced halftone dots through control of the electricalconductivity of the exposed and unexposed areas of the photohardenableelectrostatic master. Since the voltage retained by the halftone dots isalmost linearly related to the percent dot area, the thickness of theliquid electrostatic developer will be constant everywhere on the image,independent of the particular dot pattern to be developed. Other usesfor the photohardenable master include preparation of printed circuitboards, resists, soldermask, photohardenable coatings, etc.

EXAMPLES

The advantageous properties of this invention can be observed byreference to the following examples which illustrate, but do not limit,the invention.

    ______________________________________                                        Glossary                                                                      ______________________________________                                        BINDERS                                                                       B1    Polymethyl methacrylate                                                       n = 1.25, where n is the inherent viscosity                                   Tg = 110° C. where Tg is the glass transition                          temperature                                                             B2    Methyl methacrylate resin, n = 0.18, Tg = 105° C.                B3    Ethyl methacrylate resin, n = 1.50 and Tg = 70° C.               B4    Isobutyl methacrylate resin, n = 0.64, Tg = 55° C.               B5    Ethyl methacrylate resin, n = 0.83, Tg = 63° C.                  B6    Methacrylate copolymer resin, n = 0.40, Tg = 40° C.              B7    Poly(styrene/methylmethacrylate)70/30                                         Tg = 95° C.                                                      B8    Polycarbonate, Tg = 150° C.                                      B9    Polysulfone, Tg = 190° C.                                        B10   Cycolac ® CTB acrylonitrile/butadiene/styrene,                            Tg = 80° C.-84° C.                                        MONOMERS                                                                      M1    Ethoxylated trimethylolpropane triacrylate                              M2    Trimethylolpropane triacrylate                                          M3    Glycerol propoxylylated triacrylate                                     M4    tris-(2-hydroxyethyl isocyanurate) triacrylate                          CHAIN TRANSFER AGENTS                                                         CT1   2-mercaptobenzoxazole                                                   CT2   2-mercaptobenzimidazole                                                 INITIATORS                                                                    I1    2,2',4,4'-tetrakis(o-chlorophenyl)-5,5'-bis(m, -p-                            dimethoxyphenyl)biimidazole                                             I2    2,2' -bis(o-chlorophenyl)-4,4',5,5'-tetraphenyl-                              biimidazole                                                             I3    2,2'-bis(o-chlorophenyl)-5,5'-bis(m-                                          methoxyphenyl)biimidazole                                               I4    2-ethylanthraquinone                                                    I5    Benzoin methyl ether                                                    ADDITIVES                                                                     A1    triphenylamine                                                          A2    tris-(o-methyl-p-diethylaminophenyl)methane                             A3    p-toluenesulfonic acid                                                  A4    phenidone                                                               A5    1-(2'-nitro-4',5'-dimethoxy)phenyl-1-(4-t-                                    butylphenoxy)ethane                                                     STABILIZER                                                                    S1    1,4,4-trimethyl-2,3-diazobicyclo-(3,2,2)-                                     non-2-ene-2,3-dioxide                                                   ______________________________________                                    

Except as indicated otherwise, the following procedures were used in allexamples.

A solution containing about 80 parts methylene chloride and 20 parts ofsolids was coated onto a 0.004 inch (0.0102 cm) aluminized polyethyleneterephthalate support. After the film had been dried at 60-95° C. toremove the methylene chloride, a 0.00075 inch (0.0019 cm) polypropylenecover sheet was laminated to the dried layer. The coating weights variedfrom 80 to 150 mg/dm². The film was then wound on rolls until exposureand development occurred.

The formulations were tested for electrical properties as a function ofambient conditions. The environmental stability was evaluated bymeasuring the shift in transit times (a_(T)) of each material withtemperature (T) and relative humidity (RH), where the transit time τ isthe time interval required by the charge carriers to travel across thesample and reach the ground plane of the material.

It has been found that by plotting the voltage decay or discharge curvesof the electrostatically charged photohardenable master at differentenvironmental conditions noted below the curves are related to eachother. If both voltage and time are plotted as log (t) (x axis) and log(V) (y axis) then the discharge curves at differing environmentalconditions can be superimposed by horizontal shifts along the log (time)axis. The time dependence of the voltage is expressed as:

    V(t)=f(t/τ)

where τ is the transit time for any given environmental condition. τ isdependent upon the environmental conditions, i.e., temperature andhumidity, but the function f(t/τ) is invariant. As a result τ definesthe changes in the discharge characteristics within the specifiedenvironmental conditions. The shift factor a_(T) =τ₁ /τ₂ wherein τ₁ andτ₂ are the longer and shorter discharge times for two differingenvironmental conditions. a_(T) provides a direct and straight forwardway of comparing the relative humidity and temperature response of thedifferent formulations. A smaller number for a_(T) indicates lowerenvironmental sensitivity. a_(T) is 15 or less, preferably 10 or less.

The formulations were tested for the change in discharge rate (i.e.,shift in transit time (a_(T))) at the specified ambient conditions. Theenvironmental specifications were set to be the standard operatingconditions of print shops worldwide, i.e., 30%≦relative humidity(RH)≦60% and 60° F. (15.6° C.)≦temperature (T)≦80° F. (26.7° C.). Theelectrostatic setup was placed in a small Tinney Benchmark environmentalchamber which permitted accurate control of the environment in the T andRH range of interest (Tenney Engineering, Inc., South Brunswick, N.J.).

Surface voltage measurements were carried out as follows: 1 inch by 0.5inch (2.52 cm by 1.27 cm) samples were mounted on a flat aluminum platethat was positioned on a friction free translational stage connected toa solenoid. The samples were moved from position A to B, about 1 inch(2.54 cm) apart, by activating the solenoid. In position A, they wereplaced directly under a scorotron for charging. The charging conditionswere: 100-500 V grid voltage (Vg), 100-1000 microamps corona current(4.35 to 5.11 kV) and 2 seconds charging time. After charging wascomplete, the solenoid was energized and the samples moved to B, awayfrom the scorotron and directly under Isoprobe electrostaticmultimeters, Model #174, manufactured by Monroe Electronics,Lundonville, N.Y. The outputs from the multimeters were fed into acomputer (Model #9836, manufactured by Hewlett Packard, Palo Alto,Calif.) through a data acquisition box (Model #3852A, manufactured byHewlett Packard, Palo Alto, Calif.) where the voltage versus time wasrecorded for each sample. Since movement of the samples took about 1second, the "zero time" measurement was made within about 1 second aftercharging.

In order to test the image quality of each photopolymerizablecomposition, the photopolymerizable layer was exposed, charged, andtoned with magenta toner, and the image transferred to paper asdescribed below. In all cases "magenta toner" refers to the standardmagenta toner used to form a four color proof described below. Theevaluation of image quality was based on dot range and dot gain onpaper. The standard paper is 60 lbs Solitaire® paper, offset enameltext, Plainwell Paper Co., Plainwell, Mich. However, the variety ofpapers tested included: 60 lbs Plainwell offset enamel text, 70 lbsPlainwell offset enamel text, 150 lbs white regal Tufwite® Wet StrengthTag, 60 lbs White LOE Gloss Cover, 70 lbs white Flokote® Text, 60 lbswhite all purpose lith, 110 lbs white Scott index, 70 lbs white NekoosaVellum Offset and 80 lbs white Sov® text. Results indicated that,although the process can be used with any paper, the trapping of inkvaries with the fibrillar nature of the paper in use.

Dot gain or dot growth versus dot size is a standard measure of howtolerances between a proof and a press proof are determined. The dotgains were measured using designed patterns called Brunner targets whichare available from System Brunner USA, Inc., Rye, N.Y. Typically desireddot gains for graphic arts applications are in the range of 15 to 22% atmidtone. The dot range was easily tested using URGA targets, GraphicArts Technical Foundation, Pittsburgh, Pa., that include 0.5% highlightdots to 99.5% shadow dots and in a 133 lines/mm screen that includes 4μm highlights and shadow microlines. Typically desired dot ranges forgraphic arts applications in the range of 2 to 98%.

The photohardenable electrostatic master was first exposed through aseparation negative using a Douthitt Option X Exposure Unit (DouthittCorp., Detroit, Mich.), equipped with a model TU 64 Violux®5002 lampassembly (Exposure Systems Corp., Bridgeport, Conn.) and model No. 5027photopolymer type lamp. Exposure times varied from 1-100 secondsdepending on the formulation. The exposed master was then mounted on adrum surface. SWOP (Specification Web Offset Publications) density inthe solid regions was obtained by charging the fully exposed regions ofthe photopolymerizable of the electrostatic master to 100 to 200 V. Thecharged latent image was then developed with a liquid electrostaticdeveloper, or toner, using a two roller toning station and the developerlayer properly metered. The developing and metering stations were placeda 5 and 6 o'clock respectively. The toner image was corona transferredonto paper using 50-150 microamps transfer corona and 4.35 to 4.88 kV,and -2.5 to -4.0 kV tackdown roll voltage at a speed of 2.2inches/second (5.59 cm/second) and fused in an oven for 10 seconds at100° C.

The dot gain curves were measured using a programmable MacBethdensitometer, Model #RD 918, (McBeth Process Measurements, Newburgh,N.Y.) interfaced to a Hewlett Packard Computer, Model #9836. The dotgain curve was calculated by using a simple algorithm that included theoptical density of the solid patch, the optical density of the paper(gloss) and the optical density of each percent dot area in the Brunnertarget.

A four color proof is obtained by following the steps described below.First, complementary registration marks are cut into thephotopolymerizable layers of the electrostatic masters prior toexposure. Masters for each of the four color separations are prepared byexposing four photopolymerizable elements having coversheets to one ofthe four color separation negatives corresponding to cyan, yellow,magenta and black colors. Each of the four photopolymerizable layers isexposed for about 3 seconds using the Douthitt Option X Exposure Unitdescribed above. The visible radiation emitted by this source issuppressed by a UV light transmitting, visible light absorbing Kokomo®glass filter (No. 400, Kokomo Opalescent Glass Co., Kokomo, Ind.). Thecover sheets are removed, and each master is mounted on thecorresponding color module drum, in a position assuring imageregistration of the four images as they are sequentially transferredfrom each master to the receiving paper. The leading edge clamps arealso used to ground the photopolymer aluminized backplane to the drum.The masters are stretched by spring loading the trailing edge assuringthat each lays flat against its drum.

Each module comprised a charging scorotron at 3 o'clock position, adeveloping station at 6 o'clock, a metering station at 7 o'clock and acleaning station at 9 o'clock. The charging, developing, and meteringprocedure is similar to that described above prior to the examples. Thetransfer station consists of a tackdown roll, a transfer corona, paperloading, and a positioning device that fixes the relative position ofpaper and master in all four transfer operations.

In the preparation of the four-color proof the four developers, ortoners, have the following compositions:

    ______________________________________                                        INGREDIENTS             AMOUNT (g)                                            ______________________________________                                        BLACK                                                                         Copolymer of ethylene (89%) and                                                                       2,193.04                                              methacrylic acid (11%), melt                                                  index at 190° C. is 100, Acid No. is 66                                Sterling NF carbon black                                                                              527.44                                                Heucophthal Blue, G XBT-583D                                                                          27.76                                                 Heubach, Inc., Newark, NJ                                                     Basic Barium Petronate ®,                                                                         97.16                                                 Witco Chemical Corp., New York, NY                                            Aluminum tristearate, Witco 132                                                                       27.76                                                 Witco Chemical Corp., New York, NY                                            L, non-polar liquid     13,047.0                                              having a Kauri-Butanol value                                                  of 27, Exxon Corporation                                                      CYAN                                                                          Copolymer of ethylene (89%) and                                                                       3,444.5                                               methacrylic acid (11%), melt                                                  index at 190° C. is 100, Acid No. is 66                                Ciba-Geigy Monarch Blue X3627                                                                         616.75                                                Dalamar ® Yellow YT-858D Heubach, Inc.,                                                           6.225                                                 Newark, NJ                                                                    Aluminum tristearate, as described                                                                    83.0                                                  in black developer                                                            Basic Barium Petronate ®                                                                          311.25                                                (Witco Chemical Corp.)                                                        L as described in       16,60.0                                               black developer                                                               MAGENTA                                                                       Copolymer of ethylene (89%) and                                                                       4,380.51                                              methacrylic acid (11%), melt                                                  index at 190° C. is 100, Acid No. is 66                                Mobay RV-6700, Mobay Chemical Corp.,                                                                  750.08                                                Haledon, NJ                                                                   Mobay RV-6713, Mobay Chemical Corp.                                                                   750.08                                                Haledon, NJ                                                                   Aluminum tristearate, as                                                                              120.014                                               described in black developer                                                  Triisopropanol amine    75.008                                                Basic Barium Petronate ®                                                                          720.08                                                (Witco Chemical Corp.)                                                        L as described in       32,540.0                                              black developer                                                               YELLOW                                                                        Copolymer of ethylene (89%) and                                                                       1,824.75                                              methacrylic acid (11%), melt                                                  index at 190° C. is 100, Acid No. is 66                                Yellow 14 polyethylene flush,                                                                         508.32                                                Sun Chemical Co., Cincinnati, OH                                              Aluminum tristearate, as described                                                                    46.88                                                 in black developer                                                            Basic Barium Petronate ®                                                                          59.5                                                  (Witco Chemical Corp.)                                                        L as described          11,570.0                                              in black developer                                                            ______________________________________                                    

First, the cyan master is charged, developed and metered. The transferstation is positioned and the toned cyan image transferred onto thepaper. After the cyan transfer is completed, the magenta master iscorona charged, developed and metered, and the magenta imagetransferred, in registry, on top of the cyan image. Afterwards, theyellow master is corona charged, developed, and metered, and the yellowimage is transferred on top of the two previous images. Finally theblack master is corona charged, developed, metered, and the toned blackimage transferred, in registry, on top of the three previouslytransferred images. After the procedure is completed, the paper iscarefully removed from the transfer station and the image fused for 15seconds at 100° C.

The parameters used for preparation of the proof are: drum speed, 2.2inches/second (5.588 cm/second); grid scorotron voltage, 100 to 400 V;scorotron current 200 to 1000 microamps (5.11 to 6.04 kV); metering rollvoltage, 20 to 200 V; tackdown roll voltage, -2.5 to -5.0 kV; transfercorona current, 50 to 150 microamps (4.35 to 4.88 kV); metering rollspeed, 4 to 8 inches/second (10.16 to 20.32 cm/second.); metering rollgap, 0.002 to 0.005 inch (0.51 to 0.0127 mm); developer conductivity 12to 30 picomhos/cm; developer concentration, 1 to 1.5% solids.

EXAMPLE 1

Solutions of photopolymerizable compositions were prepared containing 80parts of methylene chloride and 20 parts of solids. The solids comprisedmonomer or combination of monomers, binder or combinations of binders,initiator and chain transfer agent. The solutions were coated on 0.004inch (0.0102 cm) aluminized polyethylene terephthalate support and a0.00075 inch (0.001905 cm) polypropylene cover sheet. The coatingweights varied from 80 to 150 mg/cm² or an approximate thickness of 7 μmto 12 μm is sample thickness.

The photopolymerizable layer for each element had the followingcomposition wherein the amounts are in percentages by weight.

    ______________________________________                                        Amount (%)                                                                    SAMPLE      M3     M2       I1  CT1    B7  B3                                 ______________________________________                                        CONTROL 1   24     10       5   3      58                                     CONTROL 2   22     10       5   3          60                                 EXAMPLE 1   20     13       5   3      39  20                                 ______________________________________                                    

Results are shown in Table 2 below.

                  TABLE 2                                                         ______________________________________                                               SAMPLE   a.sub.T                                                       ______________________________________                                               CONTROL 1                                                                              20                                                                   CONTROL 2                                                                              22                                                                   EXAMPLE 1                                                                               6                                                            ______________________________________                                    

This table illustrates improved (less) T/RH sensitivity of Example 1(mixed binders) versus the controls (single binder).

EXAMPLE 2

Six photopolymerizable elements were prepared and tested as described inExample 1 with the following exceptions: the photopolymerizable layerfor each element had the composition shown in Table 3 below. Results arealso shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        Amount (%)                                                                    #   M3     M1     M2   M4   I1   CT1  CT2  B7  B3  a.sub.T                    ______________________________________                                        1   30                      5    3         43  19    8.5                      2          28               5    3         35  19  12                         3                 36        5    3         37  19  10                         4                      45   5    3         30  17   4                         5   32                      6         2    37  20  14                         6   35                      5         3    30  17  15                         ______________________________________                                    

All a_(T) 's (transit times) are improvements over Controls 1 and 2 ofExample 1.

EXAMPLE 3

Six photopolymerizable elements were prepared and tested as described inExample 1 with the following exceptions: the photopolymerizable layerfor each element had the composition shown in Table 4 below. Results arealso shown in Table 4.

                  TABLE 4                                                         ______________________________________                                        Amount (%)                                                                    #   M3     M1     M2   I1  I2  I4  I5  CT1  B7  B3    a.sub.T                 ______________________________________                                        1                 30           10      3    32  25    9                       2   28                          5      3    44  20    11                      3          27                      5   3    45  20    9                       4   30                             7.5 3    40    19.5                                                                              4                       5   30                     2           3    46  20    10                      6          28          3               3    46  20    7                       ______________________________________                                    

All a_(T) 's are improvements over Controls 1 and 2 of Example 1.

EXAMPLE 4

Fifteen photopolymerizable elements (three were controls) were preparedand tested as described in Example 1 with the following exceptions: thephotopolymerizable layer for each element had the composition shown inTable 5 below. Results are also shown in Table 5.

                                      TABLE 5                                     __________________________________________________________________________    AMOUNT %                                                                      # M2  M3  I1                                                                              CT1                                                                              B1                                                                              B3  B4                                                                              B5                                                                              B6 B7  B8                                                                              B9 B10                                                                              a.sub.T                               __________________________________________________________________________    C1                                                                              10  24  5 3               58          28                                    C2                                                                              10  26  5 3                   56      20                                    C3                                                                              10  23  5 3  59                       40                                     1                                                                              10  20  5 3          20   42          9                                      2                                                                              10  20  5 3        20     40          9                                      3                                                                              12  20  5 3    20         40          8                                      4                                                                              10  20  5 3               42          9                                      5                                                                                8.3                                                                               16.6                                                                            5 3            44.7                                                                               22.4        9.5                                  6                                                                              11.4                                                                                22.6                                                                            5 3      19.3           38.7  6                                      7                                                                                8.2                                                                             16  5 3            45.2        22.6                                                                             9                                      8                                                                              14  20  5 3    20             38        8.5                                  9                                                                              12  20  5 3          22       40      9                                     10                                                                              16  12  5 3    20             34      10                                    11                                                                              36  --  5 3  37                                                                              19                     10                                    12                                                                              10  210 5 3  42                                                                              20                     9                                     __________________________________________________________________________

EXAMPLE 5

Fifteen photopolymerizable elements were prepared and tested asdescribed in Example 1 with the following exceptions: Thephotopolymerizable layer for each element had the composition shown inTable 6 below. Results are also shown in Table 6.

                                      TABLE 6                                     __________________________________________________________________________                                                            Dot Dot                                                                       Range                                                                         Gain                  # M3 M2  M1  I1 CT1 A3  A2  A1  B7  B1                                                                              B3  B4                                                                              B5                                                                              B8                                                                              A4 A5                                                                              aT (%)  (%)              __________________________________________________________________________     1                                                                              27.5       1.5                                                                              3   2   2   5       40    19         8  1-98                                                                              17                 2                                                                              27.5       1.5                                                                              3   2   2   5       40      19       8  0.5-97                                                                        16                     3                                                                              27.5       1.5                                                                              3   2   2   5       40                                                                              19             8  1-98 16                4                                                                              27.5       1.5                                                                              3   2   2   5   40    19             6  3-98  9                5                                                                              28         1.5                                                                              3   2   2   5   39    10  10         5  1-98 20                6                                                                              27.5       1.5                                                                              3   2   2   5   20  20                                                                               9  10         8  3-99 11                7                                                                              20   8.5   1    3.5                                                                             2   2     4.5                                                                             20  20                                                                               8  10         8  2-98 13                8   30      3    0.1                                                                             2   2   5   40      17.9         7  0.5-97                                                                        22                     9                                                                              16 8       5    3.15                                                                              2.5                                                                               2.5                                                                               3.5                                                                             46    13        0.05                                                                             0.3                                                                             6  2-98 16               10                                                                              19 8       2  1     3.5                                                                               3.5                                                                             4   45        14         8  2-96 18               11                                                                              19 9       2  1   3   3   4   45        14         6  2-97 16               12                                                                              17.2                                                                               8.08                                                                            4.04                                                                               2.02                                                                              3.03                                                                              3.03                                                                              3.25                                                                              43.85                                                                             15.2          0.05                                                                             0.3                                                                             5  2-96 16               13       30  1  2   3   3   4               13                                                                              44     9  1-97 22               14       30  1  2   3   3   4         14      43       9.5                                                                            1-98 18               15       28  1  2   3   3   4             13  44       7.5                                                                            1-98 14               __________________________________________________________________________

EXAMPLE 6

This example illustrates the use of the photohardenable electrostaticmaster to prepare a four color proof.

The following composition was prepared from the indicated ingredients inparts:

    ______________________________________                                        M3   M2     I1     B7   B3   CT1  A1   A2  A3   A4   A5                       ______________________________________                                        17.2 8.1    4      43.83                                                                              15.15                                                                              2    3.2  3   3    0.05 0.3                      ______________________________________                                    

After the solution was stirred for 24 hr to properly dissolve all thecomponents, it was coated onto aluminized polyethylene terephthalate at150 ft/min (45.7 m/min) coating speed. Coating weight was 130 mg/dm². Apolypropylene cover sheet was placed on the photopolymer surfaceimmediately after drying. The material thus formed was cut into fourpieces about 30 inch by 40 inch (76.2 cm by 101.6 cm) for preparation ofa four color proof.

A four color proof was obtained by following the general procedure formaking a four color proof outlined above using cyan, magenta, yellow andblack photohardenable electrostatic masters.

What is claimed is:
 1. A xeroprinting process comprising(A) exposingimagewise to actinic radiation a photohardenable electrostatic mastercomprising(1) an electrically conductive substrate, and (2) a layer ofphotohardenable composition consisting essentially of(a) at least twoorganic polymeric binders, (b) at least one monomeric compound having atleast one ethylenically unsaturated group, and (c) a photoinitiator orphotoinitiator system that activates polymerization of the ethylenicallyunsaturated monomer upon exposure to actinic radiation, thephotohardenable composition containing at least one binder having a Tggreater than 80° C. and at least one binder having a Tg less than 70° C.such that the shift in transit time (a_(T)) of the photohardenable layerin the range 30%≦relative humidity≦60% and 60° F. (15.6° C.)≦temperature≦80 F. (26.7° C.) is 15 or less, (B) charging thephotohardenable master electrostatically, (C) applying an oppositelycharged electrostatic toner, and (D) transferring the toned image to areceptor surface.
 2. A process according to claim 1 wherein the exposingradiation is modulated by digital means.
 3. A process according to claim2 wherein the digital means is a computer-controlled, visiblelight-emitting laser.
 4. A process according to claim 1 wherein theexposing radiation is modulated by analog means.
 5. A process accordingto claim 4 wherein the analog means is a line or halftone negative orpattern interposed between the radiation source and the photohardenableelectrostatic master.
 6. A process according to claim 1 wherein theelectrostatic charging is by corona discharge.
 7. A process according toclaim 1 wherein the oppositely charged electrostatic toner is present inan electrostatic liquid developer.
 8. A process according to claim 7wherein the electrostatic liquid developer consists essentially of(a) anonpolar liquid present in major amount (b) thermoplastic resinparticles, the resin particles having an average particle size of lessthan 10 μm, (c) a nonpolar liquid soluble ionic or zwitterionic chargedirector compound.
 9. A process according to claim 1 wherein theoppositely charged electrostatic toner is a dry electrostatic toner. 10.A process according to claim 7 wherein the toned image is transferred toa paper receptor.
 11. A process according to claim 9 wherein the tonedimage is transferred to a paper receptor.
 12. A process according toclaim 1 wherein the layer of photohardenable composition consistsessentially of(a) poly(styrene/methyl methacrylate) and poly(ethylmethacrylate), (b) a monomeric compound selected from the groupconsisting of glycerol propoxylated triacrylate, trimethylol propanetriacrylate and mixtures thereof, and (c)2,2',4,4'-tetrakis(o-chlorophenyl)-5,5'-bis(m,p-dimethoxyphenyl)biimidazoleand 2-mercaptobenzoxazole.
 13. A process according to claim 1 whereinthe binder having a Tg greater than 80° C. is selected from the groupconsisting of acrylate and methacrylate polymers and copolymers, vinylpolymers and copolymers, polyvinyl acetals, polycarbonates,polysulfones, polyetherimides, and polyphenylene oxides.
 14. A processaccording to claim 13 wherein the binder is a methacrylate polymer orcopolymer.
 15. A process according to claim 14 wherein the binder ispoly(styrene/methyl methacrylate).
 16. A process according to claim 14wherein the binder is poly(methyl methacrylate).
 17. A process accordingto claim 13 wherein the binder is polycarbonate.
 18. A process accordingto claim 13 wherein the binder is polysulfone.
 19. A process accordingto claim 1 wherein the binder with a Tg less than 70° C. is selectedfrom the group consisting of acrylate and methacrylate polymers andcopolymers, vinyl polymers and copolymers, polyvinyl acetals,polyesters, polyuretahnes, butadiene copolymers, cellulose esters andcellulose ethers.
 20. A process according to claim 19 wherein the binderis a methacrylate polymer or copolymer.
 21. A process according to claim20 wherein the binder is poly(ethyl methacrylate).
 22. A processaccording to claim 20 wherein the binder is poly(isobutyl methacrylate).23. A process according to claim 20 wherein the binder ispoly(cyclohexyl methacrylate).
 24. A process according to claim 19wherein the binder is poly(tertiary-butyl acrylate).
 25. A processaccording to claim 1 wherein the polymeric binder component (a) ispresent in 40 to 70% by weight, the monomeric component (b) in 20 to 40%by weight, and the photoinitiator component (c) in 1 to 20% by weight,the weight percentages based on the total weight of the photohardenablecomposition.